In general, a centrifugal compressor includes an impeller provided on a rotary shaft, and a casing which defines a flow path with the impeller by covering the impeller from the outside. The flow path is formed so that a flow path area gradually decreases from an upstream side toward a downstream side. As a result, external fluid is suctioned into the casing by the rotation of the impeller, and pressure is applied to the fluid while flowing through the flow path the fluid to discharge the fluid from the casing in a high-pressure state.
In recent years, there has been an increased demand for adopting a small casing to reduce an installation space of the compressor. When adopting a small casing, an unbalance may occur in the flow rate distribution flowing into the impeller. Due to the unbalance of the flow rate, since an attack angle to the impeller increases in a region in which the axial flow velocity decreases, there is a possibility of occurrence of stalling or surging. As a result, the operation range of the compressor may be narrowed.
As a technique for avoiding such stalling, a technique described in the following Patent Literature 1 is known. In a turbocharger described in Patent Literature 1, four fan-shaped blades are provided in a compressor introduction pipe for introducing air from the outside. These blades are rotatably supported (able to be freely opened and closed) on axes extending in the radial direction of the compressor. The degree of opening of the blades is adjusted in accordance with the flow rate of the compressed air. This makes it possible to variably control the capacity of the turbocharger and to avoid stalling of the air inside and outside the compressor introduction port.
In the device described in the above-mentioned Patent Literature 1, the compressor introduction pipe extends in the same direction as the rotation axis of the impeller. That is, the external air is configured to be guided from the axial direction to the inside of the turbocharger. However, various configurations have been proposed as an intake port (intake casing) used in a centrifugal compressor.
For example, an example is known in which an intake casing extending radially outward from a part of the axis of the impeller in the circumferential direction is provided in a single-shaft multistage centrifugal compressor. More specifically, such an intake casing includes a volute extending radially outward from the vicinity of the inlet of the impeller, and an intake port formed at the radially outer end portion of the volute. Such an intake casing covers the impeller from one side in the axial direction. Furthermore, an opening communicating with the upstream side of the impeller is provided inside the intake casing. After passing through the interior of the volute, the external air taken in from the intake port flows from the outer side toward the inner side in the radial direction through the opening formed in the intake casing, and is taken into the flow path inside the impeller.
In the centrifugal compressor of this type, it is common to avoid stalling, by providing the inlet guide vanes in the vicinity of the opening of the intake casing (that is, in the vicinity of the inlet of the impeller) as members corresponding to the blades of the turbocharger of the above-mentioned Patent Literature 1.